Transparent and reflective illumination source

ABSTRACT

A head-mounted display (HMD) includes a display illuminated by one or more illumination sources. An illumination source is coupled to a partially transparent circuit board and is configured to emit light onto a compound mirror. The compound mirror is farther from an exit pupil of the HMD than the display and reflects light from the illumination source back towards the exit pupil of the HMD. Light reflected by the compound mirror is transmitted through the partially transparent circuit board onto the display, illuminating the display.

BACKGROUND

The present disclosure generally relates to head mounted displayspresenting content to users, and specifically to an illumination sourceinclusion in a head mounted display to illuminate a user's eye.

Head mounted displays (HMDs) may present various types of content tousers. For example, a HMD presents virtual environments to users. Whenpresenting content to a user via a HMD, a system may track position orgaze of a user's eye within the HMD to improve content presented to theuser. For example, content presented by the HMD may be foveated, whichreduces resolution of presented content that is outside an area wherethe user's eyes' are focused within the HMD. Similarly, resolution ofcontent within the area where the user's eyes' are focused within theHMD is increased. In another example, content outside of the area wherethe user's eyes' are focused within the HMD is more highly compressedthan content within the area where the user's eyes' are focused withinthe HMD.

To determine where a user's eyes' are focused within a HMD, a camera ofother imaging device is often included in the HMD adjacent to imagingoptics, or a hot mirror that is transparent to visible light, butreflective to infrared light used by the imaging device is positionedbetween the user's eyes and a display within the HMD. However, as fieldsof view of content resented by virtual reality, mixed reality, oraugmented reality systems increase and distances between the user's eyesand the display within the HMD decrease, conventional approaches toidentifying pupils of a user's eyes to determine the user's focal becomeincreasingly difficult. While positioning an imaging device behind thedisplay of the HMD would offset some difficulties with such conventionalapproaches, conventional backlights used with displays, such as liquidcrystal displays, scatter and refract light, preventing an imagingdevice positioned behind the display from capturing images of the user'seye suitable for identifying the pupil of the user's eye.

SUMMARY

A head mounted display (HMD) presenting content to a user includes adisplay that is illuminated by one or more reflective illuminationsources. The display and one or more reflective illumination sources areincluded in a front rigid body of the HMD that includes an exit pupil asa location where a user's eye is positioned. The display that obtainsand presents the content to the user. For example, the display is aliquid crystal display.

To illuminate the display, a reflective illumination source includes anillumination source coupled to a partially transparent circuit board, aswell as a compound mirror. The illumination source, the partiallytransparent circuit board, and the compound mirror are positionedfarther from an exit pupil of the front rigid body of the HMD than thedisplay. For example, the illumination source, the partially transparentcircuit board, and the compound mirror are positioned nearer to a frontside of the HMD than the display. Hence, from the exit pupil 330 of thefront rigid body of the HMD the illumination source, the partiallytransparent circuit board, and the compound mirror are behind thedisplay.

The illumination source is configured to emit light onto the compoundmirror. In different embodiments, any type of illumination source may beused. Example illumination sources 310 include a light emitting diode(LED), an organic light emitting diode (OLED), a laser diode, avertical-cavity surface-emitting laser (VCSEL), a super radiant source,and a reflector included in a waveguide. The illumination source iscoupled to the partially transparent circuit board, which is nearer tothe display than the illumination source. The partially transparentcircuit board includes a transparent substrate, such as glass orplastic, and circuit traces coupling components. In various embodiments,ratios of heights to widths of each circuit trace equal or exceed athreshold value.

The illumination source emits light onto the compound mirror, which isfarther from the exit pupil of the HMD than the display and ispositioned a distance from the illumination source. The compound mirroris a mirrored surface having curvature in two orthogonal axes. Invarious embodiments, the curvature in one or more axes is spherical oraspherical. In various embodiments, the compound mirror may be a metalmirror or an interference mirror. Additionally, the compound mirrortransmits certain wavelengths of light incident on the compound mirror,while reflecting other wavelengths of light incident on the compoundmirror. For example, the compound mirror reflects visible light incidenton the compound mirror, but transmits at least 50% of infrared light(e.g., light having a wavelength of 850 nm) incident on the compoundmirror. Light emitted by the illumination source is incident on thecompound mirror, which reflects the incident light to form a ray oflight that is directed towards the illumination source and the partiallytransparent circuit board. The ray of light is transmitted by thepartially transparent circuit board towards the display.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example system environment including a head mounteddisplay (HMD) presenting content, in accordance with at least oneembodiment.

FIG. 2 shows a diagram of a head mounted display (HMD), in accordancewith at least one embodiment.

FIG. 3 is a cross-section of a front rigid body of a head mounteddisplay (HMD) including an electronic display element including areflective illumination source), in accordance with at least oneembodiment.

FIG. 4 is a cross-section of a front rigid body of a head mounteddisplay (HMD) including an electronic display element including analternative reflective illumination source, in accordance with at leastone embodiment.

FIG. 5 is an example of an array of reflective illumination sourcesilluminating a display, in accordance with at least one embodiment.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

System Overview

FIG. 1 is a system environment in which a head mounted display (HMD) 100operates. In this example, the system environment includes the HMD 100,an input/output interface 140, which are each coupled to a console 130.While FIG. 1 shows a single HMD 100, a single imaging device 120, and asingle VR input/output interface 140, in other embodiments, any numberof these components may be included in the system. For example, theremay be multiple HMDs 100 each having an associated input/outputinterface 140 and being monitored by one or more imaging devices 120,with each HMD 100, input/output interface 140, and imaging device 120communicating with the console 130. In alternative configurations,different and/or additional components may also be included in thesystem environment.

The HMD 100 presents content to a user. Example content includes images,video, audio, or some combination thereof. Audio content may bepresented via a separate device (e.g., speakers and/or headphones)external to the HMD 100 that receives audio information from the HMD100, the console 130, or both. The HMD 100 shown in FIG. 1 includes anelectronic display element 102, an optics block 104, a varifocalactuation block 106, an eye tracking module 108, one or more locators110, an internal measurement unit (IMU) 112, and one or more headtracking sensors 114.

The optics block 104 directs light from the electronic display element102, further described below in conjunction with FIGS. 3 and 4, to anexit pupil for viewing by a user using one or more optical elements,such as apertures, Fresnel lenses, convex lenses, concave lenses,filters, and so forth, and may include combinations of different opticalelements. In some embodiments, one or more optical elements in opticsblock 104 may have one or more coatings, such as anti-reflectivecoatings. Magnification of the image light by the optics block 104allows the electronic display element 102 to be physically smaller, toweigh less, and to consume less power than larger displays.Additionally, magnification of the image light may increase a field ofview of the displayed content. For example, the field of view of thedisplayed content is such that the displayed content is presented usingalmost all (e.g., 150 degrees diagonal), and in some cases all, of theuser's field of view.

In various embodiments, the optics block 104 is designed to correct oneor more optical errors. Examples of optical errors include: barreldistortion, pincushion distortion, longitudinal chromatic aberration,transverse chromatic aberration, spherical aberration, comaticaberration, field curvature, astigmatism, and so forth. In someembodiments, content provided to the electronic display element 102 fordisplay is pre-distorted, and the optics block 104 corrects thedistortion when it receives image light from the electronic displayelement 102 generated based on the content.

The varifocal actuation block 106 includes a varifocal element causingthe optics block 104 to vary the focal length (or optical power) of HMD100 to keep a user's eyes in a zone of comfort as vergence andaccommodation change. In one embodiment, the varifocal actuation block106 physically changes a distance between the electronic display element102 and the optical block 104 by moving the electronic display element102 or the optical block 104 (or both). Alternatively, the varifocalactuation block 106 changes the focal length of the optics block 104 byadjusting one or more properties of one or more lenses. Exampleproperties of a lens adjusted by the varifocal actuation block include:an optical path length, an index of refraction of a lens medium, a shapeof a lens, and so forth. For example, the varifocal actuation block 106changes the focal length of the one or more lenses using shape-changingpolymer lenses, electrowetting methods with liquid lenses,Alvarez-Lohmann lenses, deformable membrane mirrors, liquid crystal(electroactive) lenses, or phase-only spatial light modulators (SLMs),or any other suitable component. Additionally, moving or translating twolenses relative to each other may also be used to change the focallength of the HMD 100. Thus, the varifocal actuation block 106 mayinclude actuators or motors that move the electronic display element 102or the optical block 104 on a track to change the distance between themor may include actuators and other components or mechanisms for changingthe properties of one or more lenses included in the optics block 104.The varifocal actuation block 106 may be separate from or integratedinto the optics block 104 in various embodiments.

In some embodiments, different states of the optics block 104 correspondto different focal lengths of the HMD 100 or to a combination of thefocal length and eye position relative to the optics block 104. Inoperation, the optics block 104 may move in a range of ˜5 mm with apositional accuracy of ˜5 μm for a granularity of around 1000 focallengths, corresponding to 1000 states of the optics block 104. Anynumber of states could be provided; however, a limited number of statesaccommodate the sensitivity of the human eye, allowing some embodimentsto include fewer focal lengths. For example, a first state correspondsto a focal length of a theoretical infinity meters (0 diopter), a secondstate corresponds to a focal length of 2.0 meters (0.5 diopter), a thirdstate corresponds to a focal length of 1.0 meters (1 diopter), a fourthstate corresponds to a focal length of 0.5 meters (1 diopter), a fifthstate corresponds to a focal length of 0.333 meters (3 diopter), and asixth state corresponds to a focal length of 0.250 meters (4 diopter).The varifocal actuation block 106, thus, sets and changes the state ofthe optics block 104 to achieve a desired focal length.

The eye tracking module 108 tracks an eye position and eye movement of auser of the HMD 100. A camera or other optical sensor inside the HMD 100captures image information of a user's eyes, and eye tracking module 108uses the captured information to determine interpupillary distance,interocular distance, a three-dimensional (3D) position of each eyerelative to the HMD 100 (e.g., for distortion adjustment purposes),including a magnitude of torsion and rotation (i.e., roll, pitch, andyaw) and gaze directions for each eye. In one example, infrared light isemitted within the HMD 100 and reflected from each eye. The reflectedlight is received or detected by the camera and analyzed to extract eyerotation from changes in the infrared light reflected by each eye. Manymethods for tracking the eyes of a user can be used by the eye trackingmodule 108. Accordingly, the eye tracking module 108 may track up to sixdegrees of freedom of each eye (i.e., 3D position, roll, pitch, and yaw)and at least a subset of the tracked quantities may be combined from twoeyes of a user to estimate a gaze point (i.e., a 3D location or positionin the virtual scene where the user is looking). For example, the eyetracking module 108 integrates information from past measurements,measurements identifying a position of a user's head, and 3D informationdescribing a scene presented by the electronic display element 102.Thus, information for the position and orientation of the user's eyes isused to determine the gaze point in a virtual scene presented by the HMD100 where the user is looking.

Based on information from the eye tracking module 108, the varifocalactuation block 106 determines a vergence depth of a user's gaze basedon the gaze point or an estimated intersection of the gaze linesdetermined by the eye tracking module 108. Vergence is the simultaneousmovement or rotation of both eyes in opposite directions to maintainsingle binocular vision, which is naturally and automatically performedby the human eye. Thus, a location where a user's eyes are verged iswhere the user is looking and is also typically the location where theuser's eyes are focused. For example, the varifocal actuation block 106triangulates the gaze lines to estimate a distance or depth from theuser associated with intersection of the gaze lines. The depthassociated with intersection of the gaze lines can then be used as anapproximation for the accommodation distance, which identifies adistance from the user where the user's eyes are directed. Thus, thevergence distance allows determination of a location where the user'seyes should be focused and a depth from the user's eyes at which theeyes are focused, thereby, providing information, such as an object orplane of focus, for rendering adjustments to the virtual scene.

In some embodiments, rather than provide accommodation for the eye at adetermined vergence depth, accommodation may be directly determined by awavefront sensor, such as a Shack-Hartmann wavefront sensor; hence, astate of the optics block 104 may be a function of the vergence oraccommodation depth and the 3D position of each eye, so the optics block104 brings objects in a scene presented by electronic display element102 into focus for a user viewing the scene. Further, vergence andaccommodation information may be combined to focus optics block 104 andto render synthetic depth of field blur.

Locators 110 are objects located in specific positions on the HMD 100relative to one another and relative to a specific reference point onthe HMD 100. A locator 110 may be a light emitting diode (LED), a cornercube reflector, a reflective marker, a type of light source thatcontrasts with an environment in which the HMD 100 operates, or somecombination thereof. Active locators 110 (i.e., an LED or other type oflight emitting device) may emit light in the visible band (˜380 nm to750 nm), in the infrared (IR) band (˜750 nm to 1 mm), in the ultravioletband (10 nm to 380 nm), some other portion of the electromagneticspectrum, or some combination thereof.

In various embodiments, locators 110 are located beneath an outersurface of the HMD 100, which is transparent to the wavelengths of lightemitted or reflected by locators 110 or is thin enough not tosubstantially attenuate the wavelengths of light emitted or reflected bylocators 110. Further, the outer surface or other portions of the HMD100 can be opaque in the visible band of wavelengths of light. Thus, thelocators 110 may emit light in the IR band while under an outer surfaceof the HMD 100 that is transparent in the IR band but opaque in thevisible band.

The inertial measurement unit (IMU) 112 is an electronic device thatgenerates fast calibration data based on measurement signals receivedfrom one or more head tracking sensors 114, which generate one or moremeasurement signals in response to motion of the HMD 100. Examples ofhead tracking sensors 114 include accelerometers, gyroscopes,magnetometers, other sensors suitable for detecting motion, correctingerror associated with the IMU 112, or some combination thereof. Headtracking sensors 118 may be located external to the IMU 112, internal tothe IMU 112, or some combination thereof.

Based on the measurement signals from the head tracking sensors 118, theIMU 112 generates fast calibration data indicating an estimated positionof the HMD 100 relative to an initial position of the HMD 100. Forexample, head tracking sensors 118 include multiple accelerometers tomeasure translational motion (forward/back, up/down, left/right) andmultiple gyroscopes to measure rotational motion (e.g., pitch, yaw, androll). The IMU 112 can, for example, rapidly sample the measurementsignals and calculate the estimated position of the HMD 100 from thesampled data. For example, the IMU 112 integrates measurement signalsreceived from the accelerometers over time to estimate a velocity vectorand integrates the velocity vector over time to determine an estimatedposition of a reference point on the HMD 100. The reference point is apoint that may be used to describe the position of the HMD 100. Whilethe reference point may generally be defined as a point in space, invarious embodiments, reference point is defined as a point within theHMD 100 (e.g., a center of the IMU 112). Alternatively, the IMU 112provides the sampled measurement signals to the console 130, whichdetermines the fast calibration data.

The IMU 112 can additionally receive one or more calibration parametersfrom the console 130. As further discussed below, the one or morecalibration parameters are used to maintain tracking of the HMD 100.Based on a received calibration parameter, the IMU 112 may adjust one ormore IMU parameters (e.g., sample rate). In some embodiments, certaincalibration parameters cause the IMU 112 to update an initial positionof the reference point to correspond to a next calibrated position ofthe reference point. Updating the initial position of the referencepoint as the next calibrated position of the reference point helpsreduce accumulated error associated with determining the estimatedposition. The accumulated error, also referred to as drift error, causesthe estimated position of the reference point to “drift” away from theactual position of the reference point over time.

The imaging device 120 generates slow calibration data in accordancewith calibration parameters received from the console 130. Slowcalibration data includes one or more images showing observed positionsof locators 110 that are detectable by the imaging device 120. Invarious embodiments, the imaging device 120 may include one or morecameras, one or more video cameras, other devices capable of capturingimages including one or more locators 110, or some combination thereof.Additionally, the imaging device 120 may include one or more filters(e.g., for increasing signal to noise ratio). The Imaging device 120 isconfigured to detect light emitted or reflected from locators 110 in afield of view of the imaging device 120. In embodiments where locators110 include passive elements (e.g., a retroreflector), the imagingdevice 120 may include a light source that illuminates some of or all ofthe locators 110, which retro-reflect the light towards the light sourcein imaging device 120. Slow calibration data is communicated from theimaging device 120 to the console 130, and the imaging device 120receives one or more calibration parameters from the console 130 toadjust one or more imaging parameters (e.g., focal length, focus, framerate, ISO, sensor temperature, shutter speed, aperture, etc.).

The input/output interface 140 is a device that allows a user to sendaction requests to the console 130. An action request is a request toperform a particular action. For example, an action request may be tostart or end an application or to perform a particular action within theapplication. The input/output interface 140 may include one or moreinput devices. Example input devices include a keyboard, a mouse, a gamecontroller, or any other suitable device for receiving action requestsand communicating the received action requests to the console 130. Anaction request received by the input/output interface 140 iscommunicated to the console 130, which performs an action correspondingto the action request. In some embodiments, the input/output interface140 may provide haptic feedback to the user in accordance withinstructions received from the console 130. For example, haptic feedbackis provided by the input/output interface 140 when an action request isreceived, or the console 130 communicates instructions to theinput/output interface 140 causing the input/output interface 140 togenerate haptic feedback when the console 130 performs an action.

The console 130 provides content to the HMD 100 for presentation to theuser in accordance with information received from the imaging device120, the HMD 100, or the input/output interface 140. In the exampleshown in FIG. 1, the console 130 includes an application store 122, atracking module 134, and an engine 136. Some embodiments of the console130 have different or additional modules than those described inconjunction with FIG. 1. Similarly, the functions further describedbelow may be distributed among components of the console 130 in adifferent manner than is described here.

The application store 132 stores one or more applications for executionby the console 130. An application is a group of instructions, that whenexecuted by a processor, generates content for presentation to the user.Content generated by an application may be in response to inputsreceived from the user via movement of the HMD 100 or via theinput/output interface 140. Examples of applications include gamingapplications, conferencing applications, video playback application, orother suitable applications.

The tracking module 134 calibrates the system environment using one ormore calibration parameters and may adjust one or more calibrationparameters to reduce error in determining position of the HMD 100. Forexample, the tracking module 134 adjusts the focus of the imaging device120 to obtain a more accurate position for observed locators 110 on theHMD 100. Moreover, calibration performed by the tracking module 134 alsoaccounts for information received from the IMU 112. Additionally, iftracking of the HMD 100 is lost (e.g., the imaging device 120 loses lineof sight of at least a threshold number of the locators 110), thetracking module 134 re-calibrates some or all of the system environmentcomponents.

Additionally, the tracking module 134 tracks the movement of the HMD 100using slow calibration information from the imaging device 120 anddetermines positions of a reference point on the HMD 100 using observedlocators from the slow calibration information and a model of the HMD100. The tracking module 134 also determines positions of the referencepoint on the HMD 100 using position information from the fastcalibration information from the IMU 112 on the HMD 100. Additionally,the tracking module 134 may use portions of the fast calibrationinformation, the slow calibration information, or some combinationthereof, to predict a future location of the HMD 100, which is providedto the engine 136.

The engine 136 executes applications within the system environment andreceives position information, acceleration information, velocityinformation, predicted future positions, or some combination thereof forthe HMD 100 from the tracking module 134. Based on the receivedinformation, the engine 136 determines content to provide to the HMD 100for presentation to the user, such as a virtual scene. For example, ifthe received information indicates that the user has looked to the left,the engine 136 generates content for the HMD 100 that mirrors or tracksthe user's movement in a virtual environment. Additionally, the engine136 performs an action within an application executing on the console130 in response to an action request received from the input/outputinterface 140 and provides feedback to the user that the action wasperformed. The provided feedback may be visual or audible feedback viathe HMD 100 or haptic feedback via the input/output interface 140.

FIG. 2 is a diagram of the head mounted display (HMD) 100, in accordancewith at least one embodiment. In this example, the HMD 100 includes afront rigid body and a band that goes around a user's head. The frontrigid body includes one or more electronic display elementscorresponding to the electronic display element 102, the IMU 112, thehead tracking sensors 114, and the locators 110. In this example, thehead tracking sensors 114 are located within the IMU 112.

The locators 110 are located in fixed positions on the front rigid bodyrelative to one another and relative to a reference point 200. In thisexample, the reference point 200 is located at the center of the IMU112. Each of the locators 110 emits light that is detectable by theimaging device 120. Locators 110, or portions of locators 110, arelocated on a front side, a top side, a bottom side, a right side, and aleft side of the front rigid body, as shown FIG. 2.

Reflective Lighting of a Display Included in a Head Mounted Display

FIG. 3 is a cross-section of a front rigid body of a head mounteddisplay (HMD) 100 including an electronic display element 102 includinga reflective illumination source. In the example of FIG. 3, theelectronic display element 102 includes a display 305 that obtainscontent from the console 130 or from another source and presents thecontent. For example, the display 305 is a liquid crystal display. Theelectronic display element 102 also includes an illumination source 310coupled to a partially transparent circuit board 315, as well as acompound mirror 320. However, in other embodiments, the electronicdisplay element 102 may include different or additional components thanthose shown in conjunction with FIG. 3.

The illumination source 310, the partially transparent circuit board315, and the compound mirror 320 are positioned farther from an exitpupil 330 of the front rigid body of the HMD 100 than the display 305.For example, the illumination source 310, the partially transparentcircuit board 315, and the compound mirror 320 are positioned nearer toa front side of the HMD 100 than the display 305. Hence, from the exitpupil 330 of the front rigid body of the HMD 100 the illumination source310, the partially transparent circuit board 315, and the compoundmirror 320 are behind the display 305.

The illumination source 310 is configured to emit light onto thecompound mirror 320. In different embodiments, any type of illuminationsource 310 may be used. Example illumination sources 310 include a lightemitting diode (LED), an organic light emitting diode (OLED), a laserdiode, a vertical-cavity surface-emitting laser (VCSEL), a super radiantsource, and a reflector included in a waveguide. However, any suitablesource of light may be used as an illumination source 310 in differentembodiments.

The illumination source 310 is coupled to the partially transparentcircuit board 315, which is nearer to the display 305 than theillumination source 310. The partially transparent circuit board 315includes a transparent substrate, such as glass or plastic, and circuittraces coupling components. In various embodiments, the circuit traceshave a high aspect ratio, with a ratio of a height of a circuit trace toa width of the circuit trace equaling or exceeding 0.25. However, inother embodiments, the ratio of the height of the circuit trace to thewidth of the circuit trace is at least 0.5. As another example, theratio of the height of the circuit trace to the width of the circuittrace is at least 1.0, while in other examples the ratio of the heightof the circuit trace to the width of the circuit trace is at least 1.5.In various embodiments, both a front surface and a rear surface of acircuit trace are highly reflective. Additionally, the partiallytransparent circuit board 315 includes one or more mounting pads to forma bond with and an electrical connection with the illumination source310. A surface of a mounting pad nearest to the display 305 is highlyreflective. Additionally, the surface of the mounting pad nearest to thedisplay 305 is flat in various embodiments.

The illumination source 310 emits light onto the compound mirror 320,which is farther from the exit pupil 330 of the HMD 100 than the display305 and is positioned a distance from the illumination source 310. Thecompound mirror 320 is a mirrored surface having curvature in twoorthogonal axes. In various embodiments, the curvature in one or moreaxes is spherical or aspherical. An aspherical surface is a sphericalsurface modified with a conic, a polynomial aspherical surface, ananamorphic surface, a Zernike surface, or a free-form surface indifferent embodiments. The compound mirror 320 may also include astructure that controllably varies an angle of rays of light reflectedby the compound mirror 320. Example structures for varying the angle ofrays of light reflected by the compound mirror 320 include periodicvariations in curvature, random structures, and coatings to controllablyscatter light.

In various embodiments, the compound mirror 320 may be a metal mirror oran interference mirror. Example interference mirrors include those madeby physical vapor deposition (PVD), sputtering, atomic layer deposition(ALD), polymer coatings, or polymer films. A coating of the compoundmirror 320 reflects at least 50% of the light incident on the compoundmirror 320 in various embodiments; in other embodiments, the coating ofthe compound mirror 320 reflects at least 90% of the light incident onthe compound mirror 320. Additionally, the compound mirror 320 transmitscertain wavelengths of light incident on the compound mirror 320, whilereflecting other wavelengths of light incident on the compound mirror320. For example, the compound mirror 320 reflects visible lightincident on the compound mirror 320, but transmits at least 50% ofinfrared light (e.g., light having a wavelength of 850 nm) incident onthe compound mirror 320. As another example, the compound mirror 320reflects visible light incident on the compound mirror 320, buttransmits at least 90% of infrared light (e.g., light having awavelength of 850 nm) incident on the compound mirror 320.

Light emitted by the illumination source 310 is incident on the compoundmirror 320, which reflects the incident light to form a ray 325 of lightthat is directed towards the illumination source 310 and the partiallytransparent circuit board 315. The ray of light 325 is transmitted bythe partially transparent circuit board 315 towards the display 305. Insome embodiments, a quarter-wave retarder is positioned between theillumination source 310 and the display 305, so the ray 325 of lighttransmitted through the partially transparent circuit board 315 passesthrough the quarter-wave retarder before reaching the display.Additionally or alternatively, a reflective polarizer 160 is positionedbetween the illumination source 310 and the display 305, so the ray 325of light transmitted through the partially transparent circuit board 315passes through the reflective polarizer before reaching the display 305.In some embodiments, both the quarter-wave retarder and the reflectivepolarizer are positioned between the display 305 and the partiallytransparent circuit board 315, so light transmitted through thepartially transparent circuit board 315 passes through the quarter-waveretarder and the reflective polarizer before reaching the display 305.

Light from the display 305 is directed towards the exit pupil 330 of thefront rigid body of the HMD 100 by the optics block 104, as furtherdescribed above in conjunction with FIG. 1. The exit pupil 330 is thelocation of the front rigid body of the HMD 100 where an eye of the useris positioned when the user is wearing the HMD 100. In variousembodiments, an image capture device, such as a camera, that is part ofan eye tracking module 108 is positioned behind the compound mirror 320(e.g., nearer to the front side of the front rigid body of the HMD 100than the compound mirror 320). The image capture device captures lighttransmitted by the compound mirror 320, such as infrared wavelengths oflight in various embodiments, and generates images of the user's eyepositioned at the exit pupil 330, as further described above inconjunction with FIG. 1.

FIG. 4 is a cross-section of a front rigid body of a head mounteddisplay (HMD) 100 including an electronic display element 102 includingan alternative reflective illumination source. In the example of FIG. 4,the electronic display element 102 includes a display 305 that obtainscontent from the console 130 or from another source and presents thecontent. The electronic display element 102 also includes anillumination source 310 coupled to a partially transparent circuit board315, as well as a compound mirror 320. However, in other embodiments,the electronic display element 102 may include different or additionalcomponents than those shown in conjunction with FIG. 4.

In the example of FIG. 4, the compound mirror 320 is included in anoptical material 400. Example optical materials 400 include an inorganicglass (e.g., soda lime or borosilicate), a polymer (e.g., a silicone),an acrylate (e.g., polymethylmethacrylate), an epoxy, a polycarbonate,and a polyolefin (e.g., such as a cyclic polyolefin). In variousembodiments, the optical material 400 has a refractive index between 1.2and 1.9, while in other embodiments the optical material 400 has arefractive index between 1.4 and 1.6. While FIG. 4 shows an embodimentwith the compound mirror 320 included in a single optical material 400,in other embodiments, an optical material 400 encloses a surface of thecompound mirror 320 nearest the illumination source 310, while adifferent, alternative, optical material is behind an additional surfaceof the compound mirror 320 farthest from the illumination source 310.For example, an array of compound mirrors 320 is formed by injectionmolding a polymer and coating a curved surface of a compound mirror 320with a suitable reflective material, and filling a surface of the arraywith a different polymer, such as a cast and ultraviolet or thermallycured resin.

In the embodiment shown by FIG. 4, the illumination source 305 isincluded in the optical material 400; however, in other embodiments, theillumination source 305 is external to the optical material 400.Alternatively, the illumination source is optically coupled to theoptical material 400. In another embodiment, a material having less thana threshold refractive index is between the illumination source 305 andthe optical material 400.

The optical material 400 has a surface 405 proximate to the illuminationsource 310 and an additional surface 410 proximate to a rear surface ofthe compound mirror 320 (i.e., a surface of the compound mirror farthestfrom the display 305). In various embodiments, the surface 405 of theoptical material 400 is parallel to the additional surface 410 of theoptical material 400. The surface 405 and the additional surface 410 areflat and smooth in various embodiments; alternatively, the surface 405and the additional surface 410 have any suitable structure to provide atleast a threshold amount of light diffusion.

FIG. 5 is an example of an array 500 of reflective illumination sourcesilluminating a display 515. The array 500 includes multiple compoundmirrors 505 that are each illuminated by a corresponding illuminationsource 510. As further described above in conjunction with FIGS. 3 and4, each compound mirror 505 is positioned a distance from acorresponding illumination source 510 that emits light onto the compoundmirror 505. Light emitted by an illumination source 510 is reflected bythe corresponding compound mirror 505 as a light ray directed towards aportion of a display 515 that is positioned on an opposite side of theillumination source 510 than the compound mirror 505.

CONCLUSION

The foregoing description of the embodiments has been presented for thepurpose of illustration; it is not intended to be exhaustive or to limitthe patent rights to the precise forms disclosed. Persons skilled in therelevant art can appreciate that many modifications and variations arepossible in light of the above disclosure.

Embodiments may include or be implemented in conjunction with anartificial reality system. Artificial reality is a form of reality thathas been adjusted in some manner before presentation to a user, whichmay include, e.g., a virtual reality (VR), an augmented reality (AR), amixed reality (MR), a hybrid reality, or some combination and/orderivatives thereof. Artificial reality content may include completelygenerated content or generated content combined with captured (e.g.,real-world) content. The artificial reality content may include video,audio, haptic feedback, or some combination thereof, and any of whichmay be presented in a single channel or in multiple channels (such asstereo video that produces a three-dimensional effect to the viewer).Additionally, in some embodiments, artificial reality may also beassociated with applications, products, accessories, services, or somecombination thereof, that are used to, e.g., create content in anartificial reality and/or are otherwise used in (e.g., performactivities in) an artificial reality. The artificial reality system thatprovides the artificial reality content may be implemented on variousplatforms, including a head-mounted display (HMD) connected to a hostcomputer system, a standalone HMD, a mobile device or computing system,or any other hardware platform capable of providing artificial realitycontent to one or more viewers.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the patent rights be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thepatent rights.

What is claimed is:
 1. A head mounted display (HMD) comprising: anelectronic display element configured to display content to a userwearing the HMD, the electronic display including: an illuminationsource coupled to a partially transparent circuit board, theillumination source configured to emit light onto a compound mirrorpositioned a distance from the illumination source; the compound mirror,which has curvature in two orthogonal axes that reflects light from theillumination source to form a ray of light directed towards theillumination source and transmitted through the partially transparentcircuit board coupled to the illumination source; and a displaypresenting content and positioned so the ray of light transmittedthrough the partially transparent circuit board is incident on thedisplay; and an optics block configured to direct light from theelectronic display element to an exit pupil of the HMD.
 2. The HMD ofclaim 1, wherein the illumination source comprises a light emittingdiode.
 3. The HMD of claim 1, wherein the compound mirror is included inan optical material having a surface proximate to the illuminationsource and an additional surface proximate to a surface of the compoundmirror farthest from the display.
 4. The HMD of claim 3, wherein thesurface is parallel to the additional surface.
 5. The HMD of claim 3,wherein the optical material has a refractive index between 1.2 and 1.9.6. The HMD of claim 3, wherein the optical material has a refractiveindex between 1.4 and 1.6.
 7. The HMD of claim 1, wherein the electronicdisplay element further includes: a quarter-wave retarder positionedbetween the illumination source and the display, so the ray of lighttransmitted through the partially transparent circuit board passesthrough the quarter-wave retarder before reaching the display.
 8. TheHMD of claim 7, wherein the electronic display element further includes:a reflective polarizer positioned between the display and the partiallytransparent circuit board, so the ray of light transmitted through thepartially transparent circuit board passes through the quarter-waveretarder and the reflective polarizer before reaching the display. 9.The HMD of claim 1, wherein the electronic display element furtherincludes: a reflective polarizer positioned between the display and thepartially transparent circuit board, so the ray of light transmittedthrough the partially transparent circuit board passes through thereflective polarizer before reaching the display.
 10. The HMD of claim1, wherein the compound mirror reflects visible light and transmitsinfrared light.
 11. The HMD of claim 10, wherein the compound mirrortransmits at least 90% of infrared light incident on the compoundmirror.
 12. The HMD of claim 1, wherein the partially transparentcircuit board comprises a transparent substrate and one or more circuittraces each having a ratio of a height of a circuit trace to a width ofthe circuit trace equaling or exceeding 1.0.
 13. The HMD of claim 12,wherein the transparent substrate comprises glass or plastic.
 14. Adevice comprising: an illumination source coupled to a partiallytransparent circuit board, the illumination source configured to emitlight onto a compound mirror positioned a distance from the illuminationsource; the compound mirror, which has curvature in two orthogonal axesthat reflects light from the illumination source to form a ray of lightdirected towards the illumination source and transmitted through thepartially transparent circuit board coupled to the illumination source;and a display presenting content and positioned so the ray of lighttransmitted through the partially transparent circuit board is incidenton the display.
 15. The device of claim 14, wherein the compound mirroris included in an optical material having a surface proximate to theillumination source and an additional surface proximate to a surface ofthe compound mirror farthest from the display.
 16. The device of claim15, wherein the surface is parallel to the additional surface.
 17. Thedevice of claim 15, wherein the optical material has a refractive indexbetween 1.4 and 1.6.
 18. The device of claim 14, wherein theillumination source comprises a light emitting diode.
 19. The device ofclaim 14, wherein the compound mirror reflects visible light andtransmits infrared light.
 20. The device of claim 19, wherein thecompound mirror transmits at least 90% of infrared light incident on thecompound mirror.